F Lecture 3: Typical Distributions of Water

IPO: A Course for Undergraduate and Graduate Majored in Oceanography and Atmosphere

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Lecture 3: Typical Distributions of Water Characteristics

in the Oceans

Introductive Physical Oceanography

杨海军（YANG Haijun）复旦大学大气与海洋科学系

Department of Atmospheric and Oceanic Sciences, Fudan University

Email: [email protected]

This powerpoint was prepared for purposes of this lecture and course only. It contains graphics from copyrighted books,

journals and other products. Please do not use without acknowledgment of these sources.

mailto:[email protected]

Lecture 3: Typical Distributions of Water Characteristics in the Oceans 2

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Readings

Emery, Talley and Pickard: Chapter 4

Key Concepts

Stratification

Thermocline

Mixed Layer

Ventilation


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3.1 Introduction

Data Collection and Analysis

in situ (within the water column) observations by ship. A typical research cruise may include a number of oceanographic stations (sometimes called hydrographic stations) at which the ship is stopped and water properties are measured from the surface to depth.

Mooring platforms. Time series plots


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3.1 Introduction

Data Collection and Analysis

Moving platforms. Merchant vessels, XBTs (expendable bathythermographs) and XCTDs (expendable conductivity-temperature probes)

Drifting surface buoys

Satellites


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Tropical Atmosphere Ocean (TAO) Array

Motivated by the 1982-1983 El Nino event.

Support from NOAA's Equatorial Pacific Ocean Climate Studies (EPOCS) program

Started in early 1984, ended in Dec 1994. The full array of nearly 70 moorings. During the 10 years, over 400 buoys were deployed on 83 cruises, using 17 different ships from 6 different countries.

The operationally measurements consist of winds, sea surface temperature, relative humidity, air temperature, and subsurface temperature at 10 depths in the upper 500 m. Five moorings along the equator also measure ocean velocity.

http://www.pmel.noaa.gov/tao/proj_over/flash/mainDOC.html

http://www.pmel.noaa.gov/tao/proj_over/flash/mainDOC.html


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3.1 Introduction

General

Most water characteristics are vertically stratified. Horizontal changes << vertical ones over the

same distance. For instance, near the equator the temperature of the water may drop from 25oC at

the surface to 5oC at a depth of 1 km, but it may be necessary to go 5,000 km north or south from

the equator to reach a latitude where the surface temperature has fallen to 5oC. The average

vertical temperature gradient ~ 5,000 times the horizontal one.


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3.1 Introduction

General

The more gradual horizontal variations are important: the horizontal density

differences actually drive the horizontal circulation, which is much stronger than

the vertical circulation.


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3.1 Introduction

General statistics

1. 75% of the total volume of the ocean water has properties between 0C and 6Cin temperature and between 34 and 35 psu in salinity.

2. 50% of the total volume of the oceans has properties between 1.3C and 3.8Cand between 34.6 and 34.7 psu.

3. Mean temperature of the world ocean is 3.5C and the mean salinity is 34.6 psu.


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3.1 Introduction

Area descriptions

“Zonal” -- the value of a property such as surface temperature may be much the

same across the ocean in the east-west (zonal) direction but may change rapidly

in the north-south (meridional) direction.

The most marked seasonal changes take place in the temperate zones

(approximately 30o to 60oN or oS).


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3.1 Introduction

Area descriptions

“Equatorial” refers to the zone within several degrees of the equator

“Tropical” refers to zones within the tropics (23oN or oS of the equator).

“Low latitude” is used when the equatorial and tropical are to be lumped together in contrast

to the high latitudes, which are near the poles, north and south.

“Subtropical” refers to mid-latitude zones poleward of the tropics.

“Polar” is used for the Arctic and Antarctic regions.

“Subpolar” refers to the region between the strictly polar conditions and those of the

temperate mid-latitudes.


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3.2 Temperature Distribution

Vertical temperature structure

Surface temperature

Upper layer temperature

Thermocline

Temporal variations of temperature in the upper layer and thermocline

Deep water temperature and potential temperature


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Vertical Temperature Structure

Three zones in the vertical: “mixed layer（混合层）” or sometimes “surface

layer”, an upper zone of 50 to 200 m thickness with potential temperatures similar

to those at the surface; “thermocline（温跃层）”, extending down to roughly

1,000 m, the potential temperature decreases rapidly; “abyssal（深水层）”,

between the thermocline and ocean bottom, potential temperature decreases slowly.

Typical temperatures at subtropical latitudes are 20C at the surface, 8C at 500 m, 5C at 1,000 m and 1 to 2C at 4,000 m.


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Fig 3.1 Typical potential temperature (C)/depth (m) profiles for the open ocean in (a) the tropical western North Pacific (5N), (b) the subtropical western and eastern North Pacific (24N), and (c) the subpolar western and eastern North Pacific (47N).


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In all regions, spring and summer warming produces a thin warm layer overlying the winter's mixed layer. In the western subtropical regions and some other regions, there are two thermoclines with a less stratified (isothermal) layer ("thermostad") between them, all within the upper 1,000 m. In Subpolar and Polar Regions where there is a low salinity surface layer, water in the upper layer can become colder than underlying water. Thus, there can be cold water in the surface layer overlying a warmer layer, which itself overlies the thermocline (still at about 500 to 1,000 m depth) and then abyssal waters underneath .


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Fig. 3.2 Surface temperature (C) of the oceans in winter (January, February, March north of the equator; July, August, September south of the equator) based on averaged (climatological) data from Levitus and Boyer (1994).

Surface Temperature


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Satellite View

There have been significant improvements in mapping SST with

the advent of satellite remote sensing, which can now produce

daily maps of SST for most of the oceans with quite high spatial

resolution.


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Satellite View

The satellite sensors that give the highest spatial resolution

(about 4 km globally and 1 km locally) measure infrared

radiation emitted from the Earth's surface, but the presence of

clouds obscures the surface by absorbing the infrared signal.


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Satellite View

The structure of ocean currents, fronts, eddies and meanders are more apparent

in these SST images than they are in the averages of ship SST measurements used

for Fig. 3.2, which covers a month over many years rather than a single month. In

addition the infrared satellite sensor sees radiation emitted from the 10 micron

“thin skin” of the ocean while the ships and buoys measure an SST somewhat

below the surface (0.5 – 3.0 m).


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Satellite View

Fig. 3.4 (a) January and (b) July of 2001 mean monthly satellite infrared sea surface temperature (C). The 5 km, 1 day resolution of the AVHRR data (as in Fig. 3.4 c) has been averaged here to 54 km and 1 month resolution.


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Satellite View

Fig. 3.4 (c) Daily SST image for Oct. 4, 1987, showing the cloud cover in gray. The spatial resolution is 5 km.


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Satellite View

Tropical Rainfall Mapping Mission (TRMM) Microwave Imager (TMI) -- another satellite instrument, that measures lower frequency emissions from the sea surface (microwaves), is now providing all-weather maps of SST, but with a 25 km resolution. Fig. 3.5 The global SST map for Oct. 23, 2003.

The TRMM satellite was designed to study rainfall measurement variability in the tropics, so its orbit is more restricted in latitude range than is the satellite that produces the infrared images of Fig. 3.4. Global maps are easily computed for a single day from the TMI since the sensor can “see” through clouds and does not suffer from the cloud contamination seen in the infrared.

The drawback with the passive microwave SST data is its lower spatial resolution of 25 km instead of 4 km with the thermal infrared. The 4 km resolution of the global thermal infrared is derived from a 1 km inherent resolution in the infrared sensor. Thus, there is a very big difference in spatial resolution between the passive microwave and thermal infrared sensors.


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“Blended” Analysis

Ship and oceanographic station data are required on an ongoing basis for calibration of all of these satellite SST measurements. For climatological time scale studies ship and buoy SSTs can be mixed with satellite SSTs, resulting in a “blended” analysis (Fig. 3.6).


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Upper Layer Temperature

Mixed Layer (混合层):

Within the upper zone of the ocean, properties are sometimes very well mixed vertically,

particularly at the end of the night (diurnal cycle) and in winter (seasonal cycle).

This layer is mixed by the wind and by cooling at the sea surface. It is unmixed by warming and

precipitation at the sea surface and by circulations within the mixed layer that move adjacent

mixed waters of different properties over each other.


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As a rule of thumb, wind-stirred mixed layers do not extend much deeper than 100 or 150 meters and can reach this depth only at the end of winter. On the other hand, infrequent vigorous cooling or evaporation at the sea surface can cause the mixed layer to deepen locally to several hundred meters, or even to more than 1,000 m in some very special subpolar and polar locations at brief times (hours) in late winter.

Mixed layers in summer may be as thin as a meter or two, overlying a set of remnant thin mixed layers from previous days with storms and thicker remnant mixed layers from winter.


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Since the mixed layer is the surface layer that connects the ocean and atmosphere, and since SST is

the main way that the ocean forces the atmosphere, observations of the mixed layer and

understanding how it develops seasonally and on climate time scales is important for modeling and

understanding climate. (SOLAS program)

http://www.uea.ac.uk/env/solas/


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Surface Ocean – Lower Atmosphere Study (SOLAS)

To achieve quantitative understanding of the key biogeochemical - physical interactions and feedbacks between the ocean and atmosphere, and of how this coupled system affects and is affected by climate and environmental change


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Mixed Layer

A common criterion: the depth at which the water is 0.125 denser than at the surface.

In tropical and mid-latitudes, the mixed layer can be based on temperature, but at higher latitudes, it is common to find a subsurface temperature maximum lying underneath a low salinity surface layer, and so a temperature-based mixed layer definition is not useful.


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Thermocline (温跃层)

Below the surface layer, temperature begins to decrease rapidly with depth. The region of higher vertical temperature gradient is called the “thermocline”.

It is often hard to define precisely the depth limits, particularly the lower limit, of the thermocline. However, in low and middle latitudes it is clear that a thermocline is present all the time at depths between 200 and 1,000 m. This is referred to as the main or permanent thermocline.


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Thermocline (温跃层)

In polar and sub-polar waters where the surface waters may be colder than the deep waters, there is often no permanent thermocline, but there is usually a permanent “halocline” (high vertical salinity gradient) and associated permanent “pycnocline” (high vertical density gradient).

There are two complementary concepts of the thermocline, one based on vertical processes only, and the other based on horizontal circulation of the waters that form the thermocline away from where they outcrop as mixed layers in winter. Both concepts are important and work together (continued to next).


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Thermocline

The vertical processes -- downward transfer of heat from the sea surface and either upwelling or downwelling.

One might expect that as the upper waters are warmest, heat would be transferred downward by diffusion despite the inhibiting effect of the stability in the pycnocline/thermocline, and that the temperature difference between the upper and lower layers would eventually disappear. However, the deeper cold waters are fed continuously from the sea surface at higher latitudes (deep and bottom water formation regions, mainly in the northernmost North Atlantic and Greenland Sea and in various regions around Antarctica).


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Vertical processes that can maintain the thermocline in a simplified one-dimensional (1-D) model.

Thermocline


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Thermocline

These deep inflows maintain the temperature difference between the warm surface waters and cold deep waters. The deep waters upwell and warm up through downward diffusion of heat. If upwelling from the very bottom layers up to near the surface occurs through the whole ocean, the upward speed would be 0.5 to 3.0 cm/day. Unfortunately these speeds are too small to measure well with present instruments and so we are not able to test the hypothesis directly.

The result of the downward vertical diffusion of heat balanced by this persistent upwelling of the deepest cold waters results in an exponential vertical profile of temperature (Stommel, 1958; Munk, 1960), which approximates the shape of the permanent thermocline.


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Thermocline

A second, more horizontal, and complementary process for maintaining the thermocline/pycnocline was suggested by Iselin (1939) and further developed by Luyten, Pedlosky and Stommel (1983). (LPS)

The waters that make up the thermocline at a given location (latitude/longitude) originate at the sea surface far away, with the colder waters coming from higher.


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Subduction --- Thermocline

Taking a northern hemisphere point of view, in subtropical regions, where the main circulation is around gyres with strong northward-flowing western boundary currents and slow southward flow across the rest of the ocean, cold surface waters come from the north. As they move south, they "subduct" beneath the warmer waters to the south (using Luyten et al.'s term, borrowed from plate tectonics).


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Ventilation Thermocline Theory

The ocean subduction process builds up the temperature, salinity

and density structure of the main pycnocline (thermocline) in the

subtropical gyres. This structure is then modified and smoothed

by the downward diffusion of heat and deep upwelling.


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Thermocline

There are both seasonal and diurnal thermocline that are not permanent.

“Seasonal thermocline” that forms only at mid-latitudes and then only in the summer. In winter it is mixed away.

At all latitudes there is a very shallow diurnal thermocline that develops with solar heating of the surface layer. This diurnal thermocline also experiences seasonal variations.


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Thermocline

Temperature and salinity are almost completely controlled by processes at the sea

surface (with the exception of weak geothermal heating at the ocean floor, which

can slightly increase water temperature).


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Thermocline

Therefore vertical variations in temperature and salinity at a given location reflect

(1) the distant sea surface location where the water at a given depth started and

(2) mixing that smoothes out differences. The process of setting properties at the

sea surface is called “ventilation (通风)" since it occurs where ocean layers

breathe, in a sense.


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Thermocline

Ventilation regions are clearly also important for air-sea gas exchange – oxygen

(O2) enters the ocean here and carbon dioxide (CO2) may leave or enter,

depending on the difference between the ocean and atmosphere's carbon dioxide.


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Temporal variations of temperature in the upper layer and thermocline

The temperature in the upper zone and into the thermocline varies seasonally, particularly in

mid-latitudes. The layer between the surface and a depth of 25 to 200 m is usually at much

the same temperature as the surface water because of mixing due to wind and waves as

described above. In winter the surface temperature is low, waves are large, and the mixed

layer is deep and may extend to the main thermocline. In summer the surface temperature

rises, the water becomes more stable, and a seasonal thermocline often develops in the

upper zone. The thermoclines are of high vertical stability (they are essentially the

pycnoclines), and separate the waters of the upper layer and deeper zones.


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Seasonal Thermocline

Fig. 3.7. Growth and decay of the seasonal thermocline at 50°N, 145°W in the eastern North Pacific as (a) vertical temperature profiles, (b) time series of isothermal contours, and (c) a time series of temperatures at depths shown.


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Annual Range of SST

Figure 3.8 shows the annual range of surface temperature over the Pacific ), based on monthly climatological temperatures from the World Ocean Atlas (WOA05) (NODC, 2005a, 2009).

Annual variations at the surface rise from 1-2C at the equator to between 5 – 10C at 40latitude in the open ocean and then decrease toward the polar regions (due to the heat required in the melting or freezing processes where sea-ice occurs). Near the coast, larger annual variations (10 -20C) occur in sheltered areas and particularly in the northwest of the northern oceans.

These annual variations in temperature decrease with depth and are rarely perceptible below 100-300 m.

The maximum temperature at the surface occurs at the end of the warming season, in August/September in the northern hemisphere, and the minimum at the end of the cooling season, during February/March.

Below the surface, the time of occurrence of the maxima and minima are delayed by as much as 2 months relative to the times at the surface.


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Fig. 3.10. (a) Potential temperature (C), (b) salinity (psu), (c) potential density (top) potential density 4 (bottom) (kg m-3) in the Atlantic Ocean at longitude 20 -25W. Data from the World Ocean Circulation Experiment


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Fig. 3.11. (a) Potential temperature (C), (b) salinity (psu), (c) potential density

(top) and potential density 4 (bottom) (kg m-3) in the Pacific Ocean at longitude 150W. Data from the World Ocean Circulation Experiment.


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Fig. 3.12. (a) Potential temperature (C), (b) salinity (psu), (c) potential density , (top) and potential density 4 (bottom) (kg m-3) in the Indian Ocean at longitude 90E. Data from the World Ocean Circulation Experiment.


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3.3 Salinity Distribution

Surface salinity

Upper layer salinity

*Intermediate depth salinity

*Deep water salinity

*Temporal variations of salinity


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Surface Salinity

Fig. 3.13. Surface salinity (psu) in winter (JFM in NH; JAS in SH) based on averaged (climatological) data from Levitus and Boyer (1994).

The surface salinity is basically zonal although not as clearly as the SST. Salinity varies from low in rainy belts in the tropics, to high in the evaporation regions of mid-latitudes, around 25N and S to low surface salinities in the rainier subpolar regions. This meridional variation is also apparent in the east-west averages of surface salinity (Fig. 3.3).


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Surface Salinity

Surface salinity range in the open ocean: 33 -- 37 psu.

Lower values occur locally near coasts where large rivers empty and in the polar regions where the ice melts.

Higher values occur in regions of high evaporation such as the eastern Mediterranean (39psu) and the Red Sea (41 psu), both due to excessive evaporation.

On average, the North Atlantic is the most saline ocean at the surface (35.5 psu), the South Atlantic and South Pacific less so (about 35.2 psu) and the North Pacific the least saline (34.2 psu).


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E-P

Fig. 3.14. Map of the difference between evaporation and precipitation (E-P) based on data from the NCEP, in cm/year.

Surface salinity is determined climatologically by the opposing effects of evaporation (increasing it) and precipitation, runoff, and ice melt (decreasing it). The salinity maxima of Figs. 3.3 and 3.13 are in the trade wind regions where the annual evaporation (E) exceeds precipitation (P), so that (E-P) is positive (Fig. 3.14). Just north of the equator, precipitation is high and surface salinity is lower because of the Intertropical Convergence Zone (ITCZ) in the atmosphere.


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Salinity and E-P

In the Atlantic the E-P map shows maxima that are close to the eastern side of the ocean basin while the surface salinity map shows maxima at these latitudes that are biased in location towards the western side of the ocean.

Similar but weaker features are seen in the Pacific. The displacement of the salinity and E-P maxima results from the circulation of the surface waters, so that salinity is highest at the end of the passage of waters through the E-P maximum.


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Upper Layer Salinity

The vertical salinity distribution cannot be summarized quite as simply as the temperature distribution. It is possible to have either high or low salinity in the warm upper layers.

Because of the less important role in dictating the density structure, salinity is a more passive tracer than temperature, reflecting the flow directions of water masses by their salinity signatures (minima or maxima).


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Upper Layer Salinity

Fig. 3.15. Typical salinity (psu) profiles for the tropical, subtropical and subpolar regions of the North Pacific.

In subtropical regions there is high salinity near the sea surface due to subtropical evaporation and a decrease downwards

In the tropics there is often slightly lower salinity at the sea surface than in the subtropics, and a sharp subsurface salinity maximum at 100 to 200 m depth, close to the top of the thermocline. This maximum comes from the high salinity surface water in the subtropics (Figs. 3.1, 3.7), which flows equatorward and downward beneath the fresher, warmer tropical surface water.

In high-latitude regions, where there is much river runoff, high precipitation and/or seasonal ice melt, there is generally low salinity at the sea surface. A rapid increase of salinity, the "halocline", lies between the upper, low-salinity water and the deeper, more saline water.


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3.4 Density Distribution

Density at the sea surface and in the upper layer

Pycnocline

Depth distribution of potential density


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Surface Density

Fig. 3.16 Surface density (; kg m-3) in winter.

The density of sea-water at the ocean surface increases from about 22 near the equator to 26 – 28 at 50-60 latitude, and beyond this it decreases slightly (Figs. 3.3 and 3.16), due to lower salinity at higher latitudes.

Surface densities at high latitudes in the Antarctic and North Atlantic are higher than in the North Pacific because the North Pacific is the freshest open ocean region.


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Surface and Upper Layer Density

Surface density determines the depth to which waters will sink as they move away from their ventilation ("outcrop") region. In late winter, surface waters reach their local density maximum as the cooling season draws to a close. Late-winter density is associated with the deepest mixed layers. As the warming season begins (March in the NH, September in the SH), the dense winter mixed layer is "capped" by warmer water at the surface. The capped winter waters move (advect) away from the winter ventilation region. If they move into a region where the winter surface waters were less dense, then they sink beneath the local surface layers, and will not be re-opened to the atmosphere during the next winter. This is a primary mechanism for moving surface waters into the ocean interior ("subduction" process).


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Longer time-scale variations in surface density can affect the amount of intermediate and deep waters that form, and the overall size of the regions that are impacted by them. During major climate changes associated with glacial/interglacial periods, surface density distributions must have been strongly altered, resulting in very different deep water distributions.


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Winter mixed layer depths vary from tens of meters to hundreds of meters, depending on the region. In the tropics, winter mixed layer depths may be less than 50 m. Winter mixed layer depths are greatest in the subpolar North Atlantic, reaching more than 1000 meters in the Labrador Sea, and in the southern hemisphere around the northern edge of the major current that circles Antarctica, at a latitude of about 50S, reaching up to about 500 m thickness.


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Pycnocline（密度跃层）

Density must increase with depth in a system in equilibrium. Ocean density does not increase uniformly with depth. There is

usually a shallow upper layer of nearly uniform density, then a layer where the density increases rapidly with depth, called the

pycnocline, analogous to the thermocline. Below this is the deep zone where the density increases more slowly with depth.

There is little variation with latitude of the deep water density. As a consequence, in high latitudes where the surface density

rises to 27 or more there is a smaller increase of density with depth than in the low latitudes and the pycnocline is weaker.


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Vertical Structure of Potential Density

Potential density distributions with depth are simpler than temperature and salinity simply because the water column must be vertically stable.

Meridional section of potential density: bowls in the upper to intermediate ocean in the subtropics, and a large upward slope towards the southern (Antarctic) end of the section. The large increase in density from equator to pole is associated with these bowls and the Antarctic isopycnal sloping. Below about 2,000 m, the total range of potential density is small, 27.6 -- 27.9 .


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Flow tends to be along surfaces of constant potential density, which are nearly horizontal. In the upper layers we can regard this as to be along surfaces of constant . Since deep ocean water is of high density, it must have formed at high latitudes where cold, high density water is found at the surface. After formation it spreads down along almost constant density surfaces. The sinking is combined with horizontal motion so that the water actually moves in a direction only slightly inclined to the horizontal.


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Salinity is important for the density structure near the sea surface at high latitudes where precipitation or ice melt create a low salinity surface layer, for instance in the Arctic, in the region next to Antarctica, and in the subpolar North Pacific and coastal subpolar North Atlantic. In shallow coastal waters, fjords and estuaries, salinity is often the controlling factor in determining density at all depths, while the temperature variations are of secondary importance.


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10-4 per C 10-4 per PSU

dSdd

Thermal expansion VS. Saline contraction

Feistel, R, 2005, Numerical implementation and oceanographic application of the thermodynamic potential of seawater. Ocean Science. 1, 9-16.


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Thermal expansion VS. Saline contraction

Levitus Global Foam Atlantic

60N EQ. 60N EQ.

TEMP (C) 5.5 27.2 9.2 27

S (PSU) 32.4 34.8 34.7 35.6

Rho (kg/cm^3) 25.56 22.5 26.85 23.17

Rho(TEMP+5,S) change

24.84 20.81 25.91 21.48

-0.72 -1.69 -0.94 -1.69

Rho(TEMP,S-2)change

23.98 21 25.29 21.66

-1.58 -1.5 -1.56 -1.57

Rho(TEMP-5,S)change

25.98 24.01 27.53 24.68

0.42 1.6 0.68 1.51

Rho(TEMP,S+2) change

27.14 24.01 28.41 24.67

1.58 1.6 1.56 1.5

1. Thermal expansion nearly doubles from

equator to North Atlantic

2. Saline contraction hardly changes from

equator to North Atlantic.

3. Saline contraction is nearly 2 times

thermal expansion in equator.

4. Saline contraction is nearly 4 times

thermal expansion in North Atlantic.

Wang et al., 2010: Seawater density variations in the North Atlantic and the Atlantic meridional overturning circulation. Climate Dynamics,

doi:10.1007/s00382-009-0560-5.


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Dissolved Oxygen

Fig. 3.18. Profiles of dissolved oxygen (µmol/kg) from the Atlantic and Pacific Oceans. Data from the World Ocean Circulation Experiment


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Fig. 3.19. Oxygen (µmol/kg) for (a) the Pacific, (b) the Atlantic and (c) the Indian Ocean. (Data from the World Ocean Circulation Experiment.)


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Questions (Due in 2-week)

1. How deep is a typical mixed layer if mixed by wind? How deep can it reach if driven by cooling?

2. What are the typical vertical temperature and salinity profiles in the subtropical and subpolar regions of the North Pacific?

3. What are the dominant regions of net evaporation in the ocean?

4. What are the key components of the Mixed Layer heat budget?

5. Is the North Atlantic Ocean saltier or fresher on average than the Pacific? Why?

6. What regions of the ocean are characterized by large differences from summer to winter? What regions have the least seasonal variability?

7. Please describe the two processes that maintain the thermocline.


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Learning about the Ocean, the Climate and the Nature

Documents

F Lecture 3: Typical Distributions of Water